This invention relates to DC to AC converters and more particularly to a voltage boost circuit for providing the required DC bus voltage for precision DC to AC solid state converters.
A typical "stand alone" precision power DC to AC solid state converter requires a higher DC supply voltage than in commonly available from the raw AC power supply prior to rectification. Furthermore, the DC supply must consist of a split DC bus with equal positive and negative bus voltages to utilize a common ground reference with the DC-AC converter, typically consisting of a half bridge switching circuit. In this way a precision AC voltage waveform can be generated (through pulse width modulation of the DC-AC inverter) exhibiting waveform symmetry for the positive and negative half cycle amplitudes above and below the commonly referenced neutral.
Prior art utilizes typically a voltage step-up transformer at the raw power AC input. The secondary winding of the transformer feeds a controlled rectifier bridge of which DC output voltage amplitude is regulated by manipulating the conduction angle of the individual thyristors. This prior art system is relatively cumbersome and costly. Phase back angle control increases circulating currents in the AC power source, generates harmonic current amplitudes, conducted and radiated noise and heat losses.
Another common method of obtaining the required DC amplitude utilizes a high speed DC to DC inverter with a voltage step-up transformer, where the secondary of this transformer is rectified and filtered. The higher DC bus amplitude obtained by this method is maintained constant by manipulating the duty cycle of the high speed inverter. While high power densities have been obtained by this method, it required the transfer of the entire power demand through the DC to DC converter. This results in poor component utilization and is therefore costly.
This invention provides an economical DC to DC transformer by simply transforming only the difference between the available raw DC power obtained through a rectifier bridge which is directly connected to the AC raw power source and the required DC bus voltage. The circuitry of this invention in effect provides two low voltage DC power supplies that are boot-strapped to the raw DC power bus to provide the required DC precision voltage amplitude.